Survey123: how to perform simple calculations using values from multiple prior questions/fields

I'm building a survey in Survey123. I want to calculate using two prior fields but cannot find examples of proper syntax. In my situation, I'd like to calculate the total crown height by substracting the height of the lower crown from the height of the tree. The example that I have seen is a tip calculation based on just one prior question/field. If anyone can offer syntax guidance relevant to simple calculations using multiple fields/questions, I'd appreciate it!